1. Field of the Invention
The present invention relates to a packet data switching apparatus that switches variable-length packets such as Internet Protocol (IP) packets.
2. Description of the Background
Recently, there has been a significant increase in data traffic including Internet data traffic. Also, there is a move to perform conventional voice communications and services such as transaction processing, which have been performed over leased lines, on the Internet for the purpose of cost reduction. To cope with this situation, high speed, large capacity, and scalable configuration are required for packet data switching apparatuses.
FIG. 21 shows the configuration of a general packet switching apparatus. A packet switching apparatus 1 comprises: plural ingress line cards 20; plural egress line cards 30; a packet switch 10 for connecting and switching input/output lines; and a controller 40 for controlling these functional blocks.
The ingress line cards 20 and the egress line cards 30 are, in some times, configured with physically identical modules.
Methods are available for scalably expanding such a packet switching apparatus so as to have larger capacity. As shown in FIG. 22, plural packet switching apparatuses 1 are mutually connected by a connection system 50 configured with large-capacity crossbar switches and the like. Alternatively, as shown in FIG. 23, packet switches are cascaded to obtain a large capacity and increase the number of user ports.
In future, in terms of applications, more demands are expected for services such as distant learning, tele-conference and contents distribution. To efficiently offer these services over a network, the packet switching apparatuses must have the multicast function.
A packet switching apparatus to support multicast is disclosed in JP-A-235991/1993 (hereinafter referred to as “reference 1”). An ATM-based multicast system disclosed by the reference 1 is shown in FIG. 24. In this system, a fixed-length packet (ATM cell) comprised of user information 100 and a cell header 101 containing ATM call identifier VPI/VCI is inputted to an switching system. When an ATM cell arrives at the switch, the destination information of the cell is retrieved from a header conversion table, using VPI/VPI 101 as key, and the destination information is added to the cell header. For a multicast cell, a bitmap tag 110 is added; for a unicast cell, a coded tag 112 is added. The bitmap tag refers to a method by which bits corresponding to all output destinations are provided and are set to “1” for desired output destinations (for example, when eight destinations exist, the destinations are displayed by 8 bits), and the coded tag refers to a method that represents an output destination by binary (for example, when eight destinations exist, the destinations are displayed by 3 bits). Each cell is added with a distribution indicator 120 to provide identification of multicast or unicast for the cell. Multicast cells are added with temporary VPI/VCI 111 and unicast cells are added with proper VPI/VCI 113. A multicast cell inputted to the switch are copied and outputted to plural output destinations according to the bitmap tag. In a following stage of the switch, a multicast cell is identified by the distribution indicator 120 and temporal VPI/VCI 111 of a multicast cell is replaced by proper VPI/VCI before it is sent to an output line.